CacheTests regression pread_test negative bytes

For this unit test doing non http requests there is a bug where the seek has no way to walk fragments. What ends up is a read of negative bytes.

To recreate:

```
./traffic_server -K -k -R 1 -r cache -T 'cache_seek'
```

Look for message:

[Mar 19 18:30:36.867] [ET_NET 10] DEBUG: (cache_seek) Read # 0 @ 7000072/1048576 for -5951496 8B03FB19AEC93B049F2D99F782797368

Note the single cache block size (104857) and negative bytes (-5951496).

NOTE: There is a chance this may be related to a problem we've spradically seen with http assets that don't seem to have a fragment table.

To help narrow this down in CacheTest.cc (past line 400) every test may be commented out except for
```
large_write_test.clone(),
pread_test.clone(),
```